Vinyl chloride polymers (PVC) are often blended with other polymers to produce compositions with superior physical properties. For example Kopchik, U.S. Pat. No. 4,255,322, discloses melt blending of imide polymers with PVC to improve the heat-distortion resistance of the PVC. Unfortunately, PVC imposes certain constraints on the melt blending process, because it degrades readily at elevated temperatures. High temperatures or long processing times at moderate temperatures introduce yellowing and other visible signs of thermal degradation, and latent degradation problems such as accelerated weathering and age-yellowing. Because imides typically become fluid only at temperatures near or above the reasonable processing limits of PVC, adequate melt blending of imides into PVC is a critical process. Processing at temperatures as high as possible to facilitate intimate blending, yet as low as possible to limit thermal degradation, creates a narrow "window" for processing variables, and recycling of polymer processed at such elevated temperatures causes the recycled polymer, heated for an excessive time as a result of the recycling, to show overt or latent signs of thermal degradation.
Use of lower temperatures to avoid thermal degradation results in the presence of relatively large particles of the imide dispersed throughout the polymer. The larger particles may appear as visible specks or as visibly apparent domains of different refractive index, neither of which are esthetically acceptable, and even if the particles are not visible, they offer discontinuities in the polymer structure to facilitate the propagation of cracks, which results in markedly degraded impact resistance, even in the presence of impact modifiers.
Interpolymerization is a known process for achieving intimate blending of otherwise immiscible polymers. For example, Gallagher, U.S. Pat. No. 3,929,933, teaches "suspension-emulsion interpolymers" of PVC and acrylic polymers as a way of intimately dispersing rubbery polymer particles with PVC to improve the impact resistance of the resulting interpolymer. However, neither this nor other references suggest such a polymerization technique might be useful with imide polymers.
Hornbaker et al, U.S. Pat. No. 4,458,046, discloses PVC blended with styrene-maleimide copolymers to increase resistance to heat deformation and flame resistance. These blends, however, suffer from the problems of other imide-PVC blends described above.
An object of the present invention is to produce a modified polyvinyl chloride having improved heat distortion, impact and processing properties. Another object of the invention is to provide a process by which the physical property improvement attributable to polyimides may be imparted to polyvinyl chloride. Still another object of the present invention is to produce a modified polyvinyl chloride from which vinyl chloride monomer may be removed readily. Other objects of the present invention will be apparent from the following disclosure.